Abstract: The present invention provides an oral pharmaceutical composition for treating insomnia, comprising lemborexant or a pharmaceutically acceptable salt thereof, wherein a normal dose of the lemborexant or pharmaceutically acceptable salt thereof is 5 to 10 mg per day, provided that a dose of the lemborexant or pharmaceutically acceptable salt thereof is 2.5 mg per day when the pharmaceutical composition is administered to a patient together with the agent capable of moderately or strongly inhibiting CYP3A, and/or a dose of the lemborexant or pharmaceutically acceptable salt thereof is 5 mg per day when the pharmaceutical composition is administered to the patient together with the agent capable of weakly inhibiting CYP3A.